In the last few years the technology of LCD monitors has finally adopted the LED backlight. There are some advantages and some improvements in these new products. First the LED backlight requires much less power than the fluorescent tubes. Second, the LED light can give a slightly better appearance to the LCD picture because of the different spectrum of colors.
In practice, the difference between the picture quality with LED backlight or with fluorescent backlight is very small. It is a matter of use. After having seen some LCD monitors with LED backlight on the shelves people do not notice them anymore. It is probably an effect related to the change. The human sight is more sensible to changes in colors and brightness, than to the absolute values of colors or brightness, so after some time we are perfectly accustomed to a kind of picture even if it has lower brightness and a narrower color range. What we are concerned about is instead the electromagnetic field and radiation.
The EMF emissions of LCD - LED monitors are lower respect to LCD monitors with fluorescent tubes, but there is still some very nasty EM radiation. First of all, the power supply is usually internal, and since it is of switching type it creates a lot of electromagnetic emissions on a wide frequency band. Even if the power supply is external, its emissions are carried by the wire and enter in the monitor circuits. Second, the LCD control board is still attached to the back of the LCD module. Third, the various voltages needed by the LED backlight and the other components are obtained by means of DC-to-DC converters which are electronic circuits that work in the same way of switching power supplies. These circuits are embedded in the monitor box, just like the power supply and the control board. Finally the LEDs are used with a pulsed current at medium frequency.
At first sight, one may think that a commercial LCD monitor with LED backlight could be modified by replacing its internal, or external, power supply with a low EMF one, but in practice this is not possible. To avoid all electromagnetic radiation from the backlight we have to power the LED with smooth DC current, like that from a battery, but to do this the LEDs must be already wired in a suitable circuit for low voltages.
Instead the industries have adopted a more complex circuit with LEDs that works with high voltage, 60 - 100 V, and the current is not constant but pulsed at a medium frequency, 10 kHz - 60 kHz. They do this because in this way the average brightness of the LEDs can be easily trimmed with a small electronic circuit, called a PWM modulator. Instead, the circuits for having a smooth DC are bigger and heavier, and could not be fitted inside a super-slim LCD monitor. Another reason for using the PWM is that there is less lost power, a simple DC circuit usually has to waste some power to achieve the desired voltage, but in practice this would be a very small power, so the true reason because they use the LEDs with a PWM circuit is the smallness of this circuits.
So in practice it is not possible, in general, to simply detach the original power supply and attach another one. Even if we discard the PWM and put the LEDs on a constant voltage, the circuit requires a high voltage, and such high voltage is dangerous, and obtaining a smooth DC voltage of such value requires a power supply even bigger and heavier than usual.
LCD monitors with LED backlight are indeed a little better than LCD monitors with fluorescent backlight for what concerns electromagnetic emissions. If one wants to buy an LCD monitor and he or she is worried about electromagnetic emissions, then the LED backlight ones are surely better.
However this products, as usual, are not designed to avoid electromagnetic emissions as more as possible. They are designed for esthetic appearance only, so they still have a noticeable amount of electromagnetic radiation, especially on the middle and high frequencies, that people who is already partially injured by electromagnetic radiation from monitors and other sources would notice immediately.
LCD monitor emissions
Test of a particular LCD monitor with LED backlight 2012